This invention is directed to an improved machine system and process for uniformly coating particulate granules with a coating material. More particularly, the invention is directed to an improved process for manufacturing attrition resistant, controlled release fertilizers by coating a particulate plant nutrient with a coating material which is chemically bonded to the nutrient, to provide a uniform coating that can be varied in thickness to effect various rates of release.
Controlled release fertilizer particles which have remarkably high resistance to attrition and a method for their preparation are disclosed in Moore, U.S. Pat. Nos. 4,711,659 and 4,804,403. According to those patents, controlled release fertilizer particles are obtained by reacting a water-soluble central mass of plant food compound containing reactive functional groups such as urea, in particulate form, with a chemical coupling agent followed by reaction with a coating material, such as a polyol, to provide a water-insoluble-polymer coating or sealing layer on the plant nutrient. The plant nutrient and sealing layer are chemically bonded to each other through the coupling agent. Specifically, the coupling agent reacts with and connects itself to functional group on a water-soluble central mass of plant nutrient to form generally a base coating having additional reactive groups. A water-insoluble coating or sealing layer then is bonded to the base coating through its reaction with the additional reactive groups on the base coat. Thereafter, multiple reacted layers of alternate applications of coupling agent and sealing layer are formed as a coating to a desired thickness. The coated fertilizer particles are highly resistant to attrition even under extreme vibration, impact and abrasion.
Although the coupling agent as described in the ""403 patent can be any material which is reactive with the functional groups of the plant nutrient and with functional groups making up the water-insoluble coating, the polyisocyanates such as diphenylmethane diisocyanate, are preferred. Further, although the water-insoluble coating material can be any of various coating materials as described in the ""403 patent, preferably the coating material is a polyol. The disclosure of the aforesaid ""659 and ""403 patents exclusively licensed to the assignee of the present application are incorporated herein by reference.
Although the aforesaid ""659 and ""403 patents provide highly advantageous slow release fertilizers, an improved system is desired to provide uniform coatings in the large quantities of fertilizers needed for commercial application.
In accordance with the improved machine system and process of the present invention, particulate plant nutrients are subjected to a pretreatment prior to reaction with a coupling agent such as an organic diisocyanate. The pretreatment subjects heated plant nutrient granules, such as urea granules, to water containing a polyol-catalyst, such as triethanolamine, and a water-soluble low molecular polyol, such as glycerine, followed by a further concurrent or sequential pretreatment with additional low molecular weight or medium molecular weight polyester polyol, which also contains a polyol-catalyst material and may contain additional water, prior to the application of excess coupling agent, such as an organic diisocyanate. This pretreatment is believed to raise the reactive functional groups of the particulate water-soluble plant nutrient at the particulate surface for enhanced molecular contact for rapid and more efficient subsequent reaction with the coupling agent. It has been found that not only is there better reaction at the surface of the nutrient with the coupling agent, but also the coating which subsequently is applied is more uniform on each granule.
Additionally, according to the present invention, a machine system is described, wherein the first surface coating on the plant nutrient is more uniformly applied, and, with it being possible to provide, in addition to the surface reacted polymer coating layer, one or more inter-layers of a second non-reactive material, such as a wax, between additional, sequentially reacted layers of the polymer coating, which are formed after the first, surface applied polymer coating, in various coating thicknesses for various controlled release duration characteristics. Specifically, it has been found that the coating components are best applied to the plant nutrient granules by injecting separately and sequentially the plurality of coating components into a cascading mass of urea granules, which are moving continuously through a horizontal rotating cylindrical drum, immediately below the surface of the granules through a plurality of injectors with the flow characteristics of each injector being separately controlled. The cascading of the granules is in a controlled pattern which evenly distributes the injected liquid coating materials onto the plant nutrient surface. Heretofore, polymer and similar coatings primarily have been applied from spray nozzles or the like, normally as a mixture of all the components of the coating, such as resin coating material dissolved in a hydrocarbon solvent. Such spraying has been found to lead to clumping of particles and uneven application. Another advantage of injecting coating materials rather than spraying is the conservation of liquid materials utilized. Thus, there is no loss of materials due to ventilating air streams which can run as high as 30% depending on the type of spraying. This not only improves the economics of the system, but further reduces environmental concerns. Further, the machine system in applying the coating materials by injection preferably utilizes individual piston pumps having individually controlled flow characteristics for each injector which injects a separate coating component. This provides for more uniform control of the application quantity and the correct chemical reaction ratio of the coating materials to the granules being coated. Since the coating materials will react fully and most rapidly when injected onto the granule surface within a stoichiometric chemical ratio range, the reaction rate is slowed, to permit uniform flow of unreacted chemicals, by injecting a triplet sequence of diisocyanate-polyol-diisocyanate. When the first two of the triplet are injected in sequence onto the granule surface, the polymerization rate is slow since the polyol is in great excess. The third injection of the triplet completes the polymerization reaction.
The invention, therefore, comprises an improved coating process and a machine system for applying the improved coating.
The advantages of the present invention will be more readily apparent from the drawing, which describes in detail a presently preferred commercial embodiment.